In accordance with higher performance of electronic information apparatuses such as a portable telephone, a notebook personal computer, and a tablet personal computer in recent years, a high performance battery for operating these electronic information apparatus for a long time by a single battery charge is desired. Also, for reducing the greenhouse gases and due to rising gasoline prices, hybrid vehicles and electric vehicles became rapidly widely used so that high power and high capacity batteries for operating the motors loaded on these vehicles are desired. As batteries fulfilling such demands, lithium batteries are mainly used currently.
As electrolytes for lithium batteries, flammable organic solvents are currently used for the reasons such as high ion conductivity, a wide potential window, and low cost. However, since the energy density of the lithium batteries are extremely high, the flammable organic solvents are not preferable in light of safety. In order to further improve the safety of the lithium batteries, flame-resistant materials are desirably used for the electrolytes of the lithium batteries. As such flame-resistant materials, inorganic solid electrolytes attract attention.
As for the inorganic solid electrolyte, there are inorganic electrolytes of amorphia such as nitride, oxide, and sulfide; and of crystalline. The followings are known as the sulfide glass solid electrolyte: a three-component glassy solid electrolyte of lithium sulfide, germanium disulfide, and lithium iodide (Patent Literature 1), and a solid electrolyte wherein lithium phosphate exists in lithium ion conductive sulfide glass represented by a general formula Li2—X (Patent Literature 2). The ion conductivities of these are at a level of 10−4 S/cm. Further, instead of the amorphia, as the crystalline material, a crystalline material having a tetrahedron basic structure of SiS4, PO4, PS4 or PN4 is being researched in hopes of high ion conductivity, and the ion conductivity in a range of 10−3 S/cm to 10−4 S/cm is reported for a Li2S—GeS2—Ga2S3 based solid electrolyte (Patent Literature 3).
Among the solid electrolytes, as a solid electrolyte with extremely high lithium ion conductivity, a sulfide solid electrolyte called sulfide thio-LISICON (thio-LISICON: LIthium SuperIonic CONductor) is known. Among them, the ion conductivity of Li3.25Ge0.25P0.75S4 is 2.2×10−3 S/cm, and is the highest among the sulfide thio-LISICON (for example, refer to Non-Patent Literature 1). Further, in order to improve the stability of electrolytes, Li—P—S based and Li—P—S—O based sulfide solid electrolytes are reported as the sulfide thio-LISICON not including a metal element other than lithium (for example, refer to Non-Patent Literatures 2 and 3).
Patent Literature 4 proposes Li—P—S based sulfide solid electrolyte having a distinguishing X-ray diffraction peak, particularly, having a peak in the vicinity of 2θ=17.90°. Also, Patent Literature 5 proposes sulfide solid electrolyte material including a composition of Li5x+2y+3P(III)yP(V)1−x−yS4, wherein 0≤x≤0.2 and 0<y≤0.3.